As an increasing number of femto cell, pico cells, relay nodes and other numerous local eNB's (LeNB's) have been deployed, the traditional network architecture where macro eNB's are predominant are evolving gradually into a network architecture, where more types of eNB's coexist, providing network coverage at more layers. In order to improve relevant performance in the network architecture where many types of eNB's coexist, the network architecture where evolved Node B's (eNB's) cooperate or are aggregated with each other has been proposed.
FIG. 1 illustrates the network architecture of a Long Term Evolution (LTE) system, wherein an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) is consisted of eNB's. An eNB communicates with a User Equipment (UE) via an air interface. There are both a control plane connection and a user plane connection between the UE and the eNB. Each UE attached to the network is served by a Mobility Management Entity (MME) connected with the eNB via an S1-MME interface. The S1-MME interface provides the UE with a control plane service.
A Serving Gateway (S-GW) is connected with the eNB via an S1-U interface, and each UE is served by a corresponding S-GW. The S1-U interface providing the LIE with a user plane service. User plane data of the UE is carried via the S1-U interface and transmitted between the S-GW and the eNB.
In the scenario where a layered network including a local eNB and a macro eNB is deployed, e.g., a possible layered network coverage environment illustrated in FIG. 2, the macro eNB can provide underlying coverage, and the local eNB can provide hotspot coverage; there is a high-speed data/signaling interface (wired or radio interface) between the local eNB and the macro eNB; and the UE can operate while being served by the macro eNB or the local eNB. When the UE connected with the macro eNB enters a coverage area of a cell corresponding to the local eNB, a part or all of data/signaling of the UR can be transferred to the local eNB, for a service available from the local eNB, taking into account a signal strength, load balancing, etc., so that the UE can access resources of both the macro eNB and the local eNB, and inter-eNB aggregation can be performed. In this scenario, multiple Radio Bearers (RB's) of the UE can be carried respectively by a macro cell controlled by the macro eNB and a local cell controlled by the local eNB, where the RB's can include a Data Radio Bearer (DRB) and a Signal Radio Bearer (SRB).
In the existing protocol, the eNB generally detects a Radio Link Failure (RLF) in the following two schemes:
In a first scheme, the eNB judges from RLF information reported by the UE whether there is an RLF occurring with the UE served by some cell; and
In a second scheme, the eNB judges by itself whether there is an RLF occurring with the UE served by some cell.
In the first scheme, the UE determines that an RLF is detected upon detecting any one of the following situations occurring and subsequently reports it to the eNB:
(1) The timer T310expires;
(2) A random access failure indicator of the media Access Control (MAC) layer is received; and
(3) An indicator of the Radio Link Control (RLC) layer, that the largest number of retransmissions has been reached, is received.
At this time, if security of the access layer has not been activated, then the UE releases a Radio Resource Control (RRC) connection and enters the RRC_IDLE state; otherwise, the UE initiates an RRC connection reestablishment procedure.
For (1), the UE detects whether there is a problem occurring with an air interface radio link by judging it against two constants N310 and N311 and the timer T310. When the UE is in the RRC_CONNECTED state, the physical layer keeps on measuring a downlink channel quality of the serving cell and transmits an “Out-of-Sync” indicator to the RRC layer upon detecting the channel quality being below some threshold or an “In-Sync” indicator to the RRC layer upon detecting the channel quality being above a threshold. When a number N310 of consecutive “Out-of-Sync” indicators have been received and none of timers T300, T301, T304 and T311 has been started, the RRC layer of the UE judges that, there is a problem temporarily occurring with the radio link quality at that time and starts the timer T310 configured to control the longest allowable period of time for the radio link quality to resume. When a number N311 of consecutive “In-Sync” indicators have been received and the timer T310 has been started, the RRC layer of the UE judges that the radio link quality has been resumed and stops the timer T310. The UE judges that there is an RLF occurring when the T310 expires.
For (2), if the MAC layer of the UE judges that the random access procedure of the UE fails, the MAC layer will indicate a random access failure accordingly. The UE determines that a radio link failure occurs therewith from the random access failure indicator from the MAC layer thereof.
For (3), if the RLC layer of the UE judges that a data packet transmitted by the UE reaches the largest number of retransmissions, the RLC layer will indicate that the largest number of retransmissions is reached. The UE determines that a radio link failure occurs therewith from the indicator from the RLC layer thereof that the largest number of retransmissions is reached.
In the second scheme, the eNB can judge by itself whether there is an RLF occurring with the UE served by some cell, for example, upon some timer expiring, from the largest number of times that some data packet is retransmitted, etc.
In summary, an RLF detected by the eNB itself typically relates to handling of RLF occurring in the same eNB cell in the existing protocol, but an action and cooperation mechanism of the macro eNB and the local eNB in the scenario where a plurality of eNB's cooperate/are aggregated (e.g., inter-eNB aggregated, etc.) has been absent. Thus it is necessary to be concerned with how the eNB handles an RLF upon detecting the RLF by itself in the network scenario with layered coverage illustrated in FIG. 2.